A recently developed type of secondary battery or rechargeable electrical conversion cell comprises: (1) an anodic reaction zone containing a molten alkali metal anode-reactant, e.g., sodium, in electrical contact with an external circuit; (2) a cathodic reaction zone containing a cathodic reactant comprising a molten liquid electrolyte, e.g., sulfur or a mixture of sulfur and a molten polysulfide, which is electrochemically reversibly reactive with the anodic reactant; (3) a solid electrolyte which functions as a cation-permeable barrier to mass liquid transfer interposed between and in contact with the anodic and cathodic reaction zones; and (4) a current collector at least one region of which is exposed to the cathodic reactant. The current collector is in electrical contact with both the cation-permeable barrier and the external circuit. As used herein the term "reactant" is intended to mean both reactants and reaction products. Hereinafter, for simplicity, this type of secondary battery or rechargeable electrical conversion cell will be referred to as a cell.
During the discharge cycle of such a cell, molten elemental alkali metal, such as sodium, surrenders electrons to an external circuit and the resulting alkali metal cations pass through the solid electrolyte. The electrons, having passed through the external circuit to the current collector, react with the liquid electrolyte, e.g., sulfur, on the surface of the current collector to form sulfide and/or polysulfide ions. Because the ionic conductivity of the liquid electrolyte is less than the electronic conductivity of the current collector, it is desirable during discharge that both the electrons and the sulfur be applied to and distributed along the surface of the current collector in the vicinity of the cation-permeable solid electrolyte. When the sulfur and electrons are so supplied, polysulfide ions can be formed near the solid electrolyte so as to be available to combine with alkali metal cations passing out of the solid electrolyte and form alkali metal polysulfide.
During the charge cycle of such a cell, when a negative potential larger than the open circuit cell voltage is applied to the anode, the opposite process occurs. Thus electrons are removed from the alkali metal polysulfide by charge transfer at the surface of the cathode current collector and are conducted through the external circuit to the anode. The alkali metal cations formed in the cathode are conducted through the liquid electrolyte and solid electrolyte to the anode where they accept the electrons from the external circuit. Because of the aforementioned relative conductivities of the ionic and electronic phases, this charging process occurs preferentially in the vicinity of the solid electrolyte and leaves behind molten elemental sulfur.
The cathode current collector of a sodium-sulfur cell as described herein includes a porous electrode generally of carbon or graphite felt in contact with a solid cathode current collector. The porous electrode may totally or partially fill the cathodic reaction zone. The solid cathode current collector may comprise a material such as a metal, graphite or a conductive glass or ceramic. It may be merely a current collecting terminal or may also serve as a container to hold the catholyte. While both the porous material and the solid cathode current collector serve to collect current in the cathode, the porous material is generally referred to as the porous electrode or cathode electrode and the solid electrode is generally referred to as the cathode current collector or, simply, the current collector. Use of the term current collector herein is meant to encompass reference to the porous electrode and/or the solid cathode current collector. If reference is intended to only the porous electrode or the solid cathode current collector, it will be called such.
As can be readily appreciated, the production of large amounts of sulfur near the surface of the solid electrolyte has a limiting effect on rechargeability. This is the case since sulfur is nonconducting and, when it covers the surfaces of the porous electrode and/or solid electrolyte, charge transfer is inhibited and consequently the charge process is greatly hindered or terminated. Thus, in order to improve the rechargeability of a cell of this type, it was believed necessary to not only supply polysulfide to the surface of the porous electrode in the vicinity of the solid electrolyte, but also to remove sulfur therefrom.
U.S. Pat. Nos. 3,811,493 and 3,980,496 both disclose Na-S cell designs which allow or promote improved mass transportation of reactants and reaction products to and from the vicinity of the solid electrolyte and the porous electrode, during both charge and discharge of the cell. For example, the improvement disclosed in U.S. Pat. No. 3,980,496 comprises designing the cathodic reaction zone of the device such that there are a plurality of channels and/or spaces within said zone which are free of porous electrode material and which are thus adapted to allow free flow of the molten cathodic reactants during operation of the device.
U.S. Pat. No. 3,976,503 discloses an improved method for recharging Na-S cells which involves maintaining a temperature gradient within the cathodic reaction zone during recharging such that the temperature of the cathodic reactants in a first region adjacent the solid electrolyte or cation-permeable barrier is sufficiently higher than the temperature of said reactants in a second region not adjacent the barrier, such that sulfur in the first region vaporizes and is transported to said second region where it condenses.
U.S. Pat. No. 3,966,493 discloses an improved cell of the type described above which exhibits increased ampere-hour capacity as the result of an improvement which comprises: (1) employing a porous conductive material which will wick both sulfur and alkali metal polysulfides and which, in different regions of the cathodic reaction zone, exhibits different degrees of wettability by said alkali metal polysulfides, the material in a region adjacent to the cation-permeable barrier being more readily wetted by the polysulfides than is the material in a region further removed from the barrier such that sulfur will boil near said barrier and condense away from it; (2) disposing the porous conductive material within the cathodic reaction zone such that it forms and encloses one of more channels which extend from the region adjacent the cation-permeable barrier outwardly into the region of the cathodic reaction zone which is further removed from the barrier; and (3) maintaining the amount of molten cathodic reactant within the cathodic reaction zone such that said channels remain free of the molten reactant and are thus adapted to transport sulfur vapor.
The devices of U.S. Pat. Nos. 3,966,492 and 3,951,689 each employ electrode materials which are preferentially wet by polysulfide salts. These patents also teach the use of such a material in conjunction with an electrode material which is preferentially wet by sulfur.
U.S. Pat. Nos. 4,084,041 and 4,084,042 are directed to Na-S cells which are taught to have increased ampere-hour capacity. The improvement of the cells comprises employing an electrode, at least a portion of which consists of a porous conductive substrate which (1) is coated, at least in part, with particularly defined materials which render the electrode portion more readily wettable by molten polysulfide than by molten sulfur, (2) does not fill the entire region of the cathodic reaction zone, and (3) is, at least in part, adjacent to and continuous with the solid electrolyte.
It was also customary to prepare a cell of this type without regard to the presence of corrodible materials or other impurities, particularly in the presence of the cathodic reactants. Thus, the cell container may have been made of metal, e.g., stainless steel, and the porous electrode may have been formed of metal, e.g., a stainless steel felt. Although such cells demonstrate excellent chargeability characteristics initially, they tend to show decreased capacity with each successive cycle. An examination of such prior art cells that have deteriorated in their charge/discharge capacity to the point of failure has shown that the shortened cycle life and deterioration of charge/discharge capacity of these cells might be attributed, at least in part, to the corrosion of the metal container or electrode, the accumulation of metal corrosion products on the solid electrolyte surface, the accumulation of corrosion products within the porous electrode and decreased mobility of sodium polysulfide within the porous electrode as a result of such corrosion product accumulation. In order to solve the problems of decreased cycle life and deteriorating charge/discharge capacity, it has been proposed to exclude metal or metal compound corrosion products from the cathodic reaction zone by forming the cathodic container and electrode from conductive, non-metallic materials, which are inherently non-corroding. An exception to this proposed exclusion of metals is the metal molybdenum, since it is substantially non-corroding in the sulfur-polysulfide catholyte. Preparing cells or batteries free of metals or metal compounds other than molybdenum was described to result in a significant increase of charge/discharge cycle life and a stabilization of charge/discharge capacity. However, such cells, including those with molybdenum current collectors, after initial discharge, do not recharge to the extent that those cells containing corrodible metal impurities do.
U.S. Pat. No. 4,002,806 teaches that the lack of ampere-hour capacity found in metal-free or non-corroding cells of the type described can be overcome. It taught that this could be accomplished by including certain additives: particulate metals, metal salts, and other metal compounds in the cathodic reactant. It is further taught therein that by including such additives in the cathodic reactant, such non-corroding cells retain long cycle life and stabilized capacity. The mechanism by which these additive materials increase ampere-hour capacity of the device is not known. One of several theories mentioned in the patent is that the materials may to an extent coat the graphite felt, thereby rendering it preferentially wettable by polysulfide and, thus, increasing charge efficiency. Such a mechanism, as stated therein, is only one of several possibilities, and is not certain. It is just as likely that, when these materials are dissolved in or mixed with polysulfide melt, they impart general or localized electronic conductivity to the melt, thereby extending the effective electrode area, altering the electrode kinetics and improving charge capacity. In that patent, however, unlike in the invention disclosed herein, the additive is incorporated in the cathodic reactant when the cell is assembled in a form whereby the entire amount of additive is immediately available to the cathodic reactants, i.e., for reaction therewith. However, we have found that doing so causes the additive to be "used up" in relatively few charge/discharge cycles. This is because during operation of the cell, the additive reacts with (i.e., is corroded by) the cathodic reactants, namely sulfur and sodium polysulfides, to form products which have low solubilities in the cathodic reactant. These corrosion products tend to migrate by diffusion and convection to regions within the cathode space electrochemically favorable to deposition. In this manner, such otherwise favorable corrosion products are depleted after the cell has been cycled a number of times, far short of the potential number of cycles of which the cell is capable. A larger amount of additive oould be initially incorporated into the cell in an attempt to provide additive which would be available for a greater number of cycles of the cell. However, such large amounts of additive would, if added in an available form as in the patent described above, have an adverse effect on the life of the cell in a relatively short time. This is because such a large initial amount of additive would: (1) consume a larger amount of the cathodic reactants making a portion of the cathode reactant unavailable for reversible electrochemical reaction: (2) form corrosion products which would extensively contaminate the solid electrolyte surface, thereby increasing its resistance to cation permeability and increasing thermal-mechanical stresses within the solid electrolyte; and (3) deposit substantial amounts of corrosion product solids within the porous cathode electrode reducing both exposed surface area as well as porous paths necessary for cathode reactant convection. Overall, the availability of the entire amount of additive at the start of the cell's operation results in increased electrochemical polarization and ohmic losses as well as the depletion of the additive in a relatively few charge/discharge cycles, resulting in loss of capacity and efficiency. Additionally, the life-time potential of the cell is further reduced by the increased stresses on the solid electrolyte caused by the impurity products. Thus, when the entire additive is available and released "in bulk" at the beginning of the cells' operation, it has deleterious effects on the extended performance of the cell. Such is the case in the invention disclosed in U.S. Pat. No. 4,002,806 wherein the additive is controlled as to kind and total amount but the additives availability to the cathode process is not controlled.
One object of this invention is to provide efficient two-phase chargeability of sodium-sulfur cells over the potential lifetime of such cells. These cells may utilize (1) substantially non-corroding cathode materials (i.e., materials which are substantially non-corroding when contacted by the cathodic reactant) such as molybdenum, graphite or carbon, conductive glass, or conductive ceramic materials or (2) slowly corroding cathode materials such as chromium plate, diffused chrome, or steel with a high chromium content.
It is another object of this invention, i.e., in addition to providing efficient two-phase chargeability, to substantially reduce the corrosion rate of cathode materials such as chromium plate which corrode slowly during the operation of standard alkaline metal-sulfur cells.
I have found that the chargeability of a Na-S battery or cell can be improved for its entire potential lifetime by providing metal or metal compound additives in a form which causes the additive not to be released all at once but rather to be released at a controlled rate and substantially continuously into the cathodic reactant during operation of the cell, preferably for the potential lifetime of the cell. I have also found that such continuous and controlled release of selected additives effectively provides protection against corrosion of cathode current collectors made of corrodible metals such as chromium or chromium steels.